1. Field of the Invention
The invention relates to an optical system for ultraviolet light having wavelengths λ≦200 nm. A preferred field of application is projection objectives for microlithography.
2. Description of the Related Art
Optical systems that can be used in the deep ultraviolet range (DUV) at wavelengths λ≦200 nm are required for example in microlithography projection exposure apparatuses for the fabrication of semiconductor components and other finely patterned devices. They may be provided as projection objectives for projecting patterns of photomasks or reticles, which are referred to generally hereinafter as masks or reticles, onto a light-sensitive object, in particular a semiconductor wafer coated with a light-sensitive layer, with a high resolution on a demagnifying scale. In order to be able to produce ever finer structures in this case, on the one hand the image-side numerical apertures NA of the projection objectives are increased further and further, and on the other hand shorter and shorter wavelengths are used, in particular ultraviolet light at λ≦200 nm, for example at 193 nm or 157 nm.
In principle, it is possible to work with purely refractive (dioptric) projection objectives, the production of which is readily controllable on account of their rotational symmetry about the optical axis. For very small resolutions, however, it is necessary in this case to work with numerical apertures NA of more than 0.8 or 0.9, which can be realized only with difficulty in dry systems with a sufficiently large working distance on the image side. Refractive immersion systems which enable values of NA>1 by the use of an immersion liquid having a high refractive index between objective exit and image plane have also already been proposed.
At wavelengths λ≦200 nm, however, there are only few sufficiently transparent materials available for producing the transparent optical elements. They include primarily synthetic quartz glass (fused silica), which is sufficiently transparent down to 193 nm, and also some fluoride crystal materials which still exhibit sufficiently low absorption even at wavelengths of 157 nm and below. In this case, primarily calcium fluoride and barium fluoride are of practical importance for the production of lenses and other optical elements; magnesium fluoride (birefringent), lithium fluoride, lithium calcium aluminum fluoride, lithium strontium aluminum fluoride or similar fluoride crystal materials are also taken into consideration for specific applications. However, since the Abbe constants of these materials are relatively close together, it becomes more and more difficult to provide purely refractive systems having sufficient correction of color errors (chromatic aberrations).
Therefore, catadioptric systems in which refractive and reflective components, in particular lenses and concave mirrors, are combined are often used for very high resolution projection objectives.
Many known catadioptric systems having one or two intermediate images have at least one concave mirror which is arranged in the region of a pupil surface of the optical system and in direct proximity to which is situated at least one negative lens. The negative lens near the pupil makes it possible to provide a chromatic overcorrection by means of which the chromatic undercorrection of other objective parts can be at least partly compensated for.
High-aperture projection objectives for microlithography, especially those for immersion operation, are intended to transport a high geometric light conductance and to provide an excellent optical correction for all beam bundles occurring therein. The geometric light conductance (also called etendue) is defined here as the product of image-side numerical aperture NA and image field size and represents a conservation variable of an optical imaging system. The conventional design approaches cannot achieve this object or can achieve it only with disproportionately high outlay. Refractive systems require a material deployment that increases exponentially with the aperture, and also aspheric forms that are difficult to manufacture. The catadioptric systems with folding mirrors for beam splitting often have structural space problems and a high material deployment may likewise be necessary.
In order to avoid the aforementioned problems, catadioptric projection objectives with at least two intermediate images have been proposed. In this case, systems having an even number of concave mirrors which, in terms of the design, are constructed rotationally symmetrically with respect to the optical axis have turned out to be advantageous for the optical correction, specifically the correction of the image field curvature (Petzval error), in conjunction with at the same time low material deployment and good constructability and alignability.
The applicant's U.S. Pat. No. 6,600,608 B1 shows a catadioptric projection objective having a first, refractive subsystem, which generates a first intermediate image, a second, catadioptric subsystem, which generates a second intermediate image from the first intermediate image, and a third, refractive subsystem, which images the second intermediate image into the image plane. The catadioptric subsystem has two concave mirrors facing one another with central holes, in the region of which lie the intermediate images. A negative meniscus lens for color correction is fitted before each of the concave mirrors. The system is well corrected with regard to chromatic longitudinal aberrations (CHL) and chromatic transverse aberrations (CHV), but has a pupil obscuration on account of the perforations of the reflective mirror surfaces.
The U.S. provisional application bearing Ser. No. 60/536,248 and with an application date of Jan. 14, 2004 shows various axially symmetrically constructed catadioptric systems with three concatenated imaging systems which image an object into an image plane whilst generating two intermediate images. These systems comprise two concave mirrors facing one another which are arranged eccentrically with respect to the optical axis and, if appropriate, alongside the optical axis and are illuminated asymmetrically. The radiation is guided past the concave mirrors laterally in a manner free of rignetting. This enables an obscuration-free imaging with extremely high numerical apertures, values of NA=1.3 or greater being attainable in conjunction with an immersion liquid. It has been shown that such advantageous systems are comparatively difficult to correct chromatically.
In order to achieve a predetermined degree of correction of chromatic aberrations in the case of purely refractive systems, from among the available materials it is necessary to find suitable material combinations for the refractive optical components. It is already known from the field of refractive optical systems to resort to liquid, transparent materials for forming liquid lenses. The U.S. Pat. No. 5,627,674 discloses chromatically corrected lens systems that are intended to permit passage of light with a small wavefront error over a wide spectral range of ultraviolet light between approximately 250 nm and approximately 450 nm. A lens system comprises a first rigid lens element, a second rigid lens element and a liquid lens arranged in an interspace formed between the lens elements. Depending on the embodiment, the fixed lenses consist of synthetic quartz glass or sapphire (Al2O3). The liquids used for the liquid lenses are, inter alia, carbon tetrachloride (CCl4), a hexane or a specific perfluoromethyldecalin in accordance with a predetermined specification. No statements are made with regard to the use of these inherently chromatically corrected liquid lens groups.
The U.S. Pat. No. 5,532,880 shows various laser beam expansion systems with lens elements made of lithium fluoride (LiF), barium fluoride (BaF) or potassium bromide (KBr) which are intended to enable a diffraction-limited beam expansion for a wide wavelength range between approximately 240 nm and 2500 nm without generation of an intermediate image. The beam expansion system contains a liquid lens group in which a spectrally pure liquid is arranged between two lenses made of the same material (lithium fluoride, barium fluoride or potassium bromide) that delimit an interspace, said liquid being a siloxane having a defined specification which is selected such that the solid materials and the liquid are inert with respect to one another.
The European patent application EP 1 524 558 A1 with application number 03256499.9 and application date Oct. 15, 2003 (corresponding to US 2005/179877 A) describes a projection system for immersion lithography. In one embodiment, the immersion liquid introduced between the last optical element of the projection objective and the substrate is utilized as a manipulator for shifting/tilting the last optical element in order to produce different focus positions of the projection radiation for so-called “focus drilling”.
Immersion lithography is possible, for example, in the deep ultraviolet range (DUV) at about 248 nm, about 193 nm or about 157 nm wavelength. Water is a frequently proposed immersion liquid due to its transparency at 193 nm. Fluoridized and siloxan-based liquids are discussed as immersion liquids for 157 nm. In international patent application WO 2005/050324 A2 published on Jun. 2, 2005 measures are described for increasing the refractive index of immersion liquids by using additives.